This document describes an experiment to generate and demodulate frequency shift keyed (FSK) signals. FSK modulation encodes digital data by shifting the carrier frequency between two discrete levels. The experiment uses MATLAB to generate an FSK signal by shifting the carrier between two frequencies (10Hz and 20Hz) according to a random binary data sequence. The FSK signal and original digital signal are plotted. FSK demodulation is performed using a phase locked loop circuit and low pass filters to recover the transmitted data from the modulated carrier.

1. EEP316 Laboratory
Frequency shift Keying (FSK)
6/2/13
Indra Bhushan 2010EE50548
Umang Gupta 2010EE50564
Vivek Mangal 2010EE50566

2. Aim: To generate and demodulate frequency shift keyed signal.
Theory:
Generation of FSK:
Amplitude shift keying - FSK - is a modulation process, which imparts transmission at
two or more discrete frequency levels. These are related to the number of levels
adopted by the digital message. For a binary message sequence there are two levels, one
of corresponds to zero and other two one.
Demodulation
FSK signal is has two distinct frequency carrier wave and hence one can use an
asynchronous demodulator. Two filters are used that are tuned to different frequencies
for demodulation. One filter detects one and another zero. The two signals are then
summed up to find the detected wave. However, we used pll circuit similar to fm for
demodulation.
MATLAB CODE:
%FSK Modulation
N=10;
d=round(rand(1,N))
fs=1000;
ts=0:1/fs:1;
fb=100;
tb=1/fb;
f1=10;
t1=1/f1;
f0=20;
t0=1/f0;
time=[];
FSK_signal=[];
Digital_signal=[];
for i=1:1:N
FSK_signal=[FSK_signal(d(i)==0)*sin(2*pi*f0*ts)+(d(i)==1)*sin(2*pi*f1*ts)];
Digital_signal=[Digital_signal(d(i)==0)*zeros(1,length(ts))+(d(i)==1)*ones(1,length(ts))];
time = [timets];
ts =ts+1;
end
% Plot theFSK Signal
subplot(2,1,1);
plot(time,FSK_signal);
xlabel('Time(bitperiod)');
ylabel('Amplitude');

3. title('FSKSignalwith two Frequencies');
axis([0time(end) -1.51.5]);
grid on;
% Plot theOriginal DigitalSignal
subplot(2,1,2);
plot(time,Digital_signal,'r','LineWidth',2);
xlabel('Time(bitperiod)');
ylabel('Amplitude');
title('OriginalDigitalSignal');
axis([0time(end) -0.51.5]);
grid on;

4. Observations:
FSK modulationusing DUAL ANALOG SWITCH

5. Comment: Here is the fft plot of FSK signal. Signal Spectrum is band limited at carrier
frequency.
FSK was DEMODULATED using pll and low pass filter.